1. Field of the Invention
This invention relates to improvements in multiple beam diode laser structures. More particularly, this invention joins two multiple beam semiconductor diode lasers into a single array for use in a laser printer.
2. Description of the Related Art
In xerographic printing, one method of forming a latent image on a photoreceptor is by raster sweeping a modulated laser beam across the charged photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image onto a recording medium, usually plain paper.
While xerographic printing has been successful, problems arise as printing speed increases and it becomes more difficult to scan the laser beam across the photoreceptor at the required speed. Scanning is commonly achieved by deflecting the laser beam from a rotating mirror. Such scanners are referred to as Raster Output Scanners (ROS). With ROSs, one way to scan the laser beam faster is by increasing the rotation speed of the mirror. However, extremely fast mirror rotation requires an expensive drive motor and an increasingly more powerful laser. A second method is to increase the raster sweep speed by using a multifaceted, rotating polygon mirror and a related set of optics. A third method is to sweep several laser beams simultaneously. These scanners are referred to as multiple beam ROSs and are a preferred method to achieve high speed scanning in printers.
Printers using multiple beam ROSs are illustrated in U.S. Pat. No. 4,253,102 to Kataoka, the disclosure of which is incorporated herein by reference. In such printers, the ROS uses a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction (also known as the fast-scan direction.) The recording medium is advanced in an orthogonal, or "process", direction (also known as the slow-scan direction) such that the beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a binary sample stream, whereby the recording medium is exposed to the image represented by the samples as it is being scanned.
Lasers arranged in the cross-scan direction must be fabricated such that they are spaced closely together in a direction parallel to the polygon mirror rotation axis to enable high density line printing. Such close spacing eliminates the need for inclining the laser array to reduce the distance among individual laser emitters as taught in the Kataoka patent. However, it is also desirable to maintain closely spaced lasing emitters so that the light beams strike as nearly the same portion of the polygon mirror as is possible and deviations in beam characteristics, e.g. spot size, energy uniformity, bow and linearity, due to off axis source positions are minimized.
The raster sweep rate problem becomes even more apparent when printing in color at high speeds. A color xerographic printer requires a separate image for each color printed. A full color printer typically requires four images, one for each of the three primary colors of cyan, magenta, yellow, and an additional one for black. Color prints are currently produced by sequentially transferring and fusing overlapped colors onto a single recording medium that is passed multiple times, once for each color, through the printer. Such printers are referred to as multipass printers.
If each color is associated with a separate photoreceptor, the printer is referred to as a multistation printer. In these printers, high speed color xerographic image output terminals require multiple independently addressable raster lines to be printed simultaneously at separate locations. Usually four independent ROSs are required. If the stations use different positions on the same photoreceptor, the printer is referred to as a single station/multiposition printer.
Multistation and single station/multiposition printers are preferred because they have greater printed page output than a multipass printer operating at the same raster sweep speed. However, problems with these systems include the high cost related to the use of multiple ROSs, the high cost of producing nearly identical multiple ROSs and the difficulty of registering (overlapping) color images on the photoreceptor. Therefore, a printer with a single ROS and a multiple diode laser array is preferable.
U.S. Pat. No. 5,243,359 by Fisli, which is incorporated herein by reference, discloses one way to construct a ROS system enabling deflection of multiple laser beams with a single ROS in a multistation printer. The rotating polygon mirror simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams having a common optical axis and substantially common origin. The clustered beams are subsequently separated by a plurality of optical filters and are directed onto associated photoreceptors of a multistation printer. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar path lengths for each beam. This is facilitated by locating all lasers in one integral unit.
However, economically feasible optical filters require the dissimilar beams to be separated by a sufficiently large wavelength. Typically a wavelength difference of about 50 nm is required. For example, U.S. Pat. No. 5,243,359 utilizes lasers emitting at 645, 695, 755, and 825 nm. Since laser emission from closely spaced monolithic laser sources over this wavelength span is not available, practical systems need to integrate a separate multiple beam diode laser for each wavelength required.
U.S. Patent Application Ser. No. 07/948,531, now U.S. Pat. No. 5,343,224, to Thomas L. Paoli, which is incorporated herein by reference, discloses an alternative multistation printer apparatus employing deflection of multiple laser beams with a single ROS. A single rotating polygon mirror simultaneously deflects a plurality of orthogonally polarized and dissimilar wavelength laser beams. The orthogonally polarized beams are subsequently separated by a polarized beam separator and a plurality of dichroic beam separators. The separated beams are directed onto their associated photoreceptors. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar path lengths for each beam. This is facilitated by locating all lasers in one integral unit. However, such a system requires the dissimilar beams to be separated by a sufficiently large wavelength difference and to emit beams that are orthogonally polarized. For example, U.S. Patent Application Ser. No. 07/948,531, now U.S. Pat. No. 5,343,224, utilizes lasers emitting at 600 and 650 nm that are orthogonally polarized. Monolithic laser sources emitting cross-polarized laser beams at substantially the same wavelength are described in U.S. Patent Application Ser. No. 07/948,524. However, since laser emissions from closely spaced monolithic laser sources emitting cross-polarized beams at wavelengths separated by 50 nm are not available, practical systems need to integrate a separate multiple beam diode laser for each wavelength required.
Accordingly, there is a need for an integrated diode laser array assembly that produces multiple, nearly coaxial laser beams emitted from closely spaced lasing elements having substantially different optical wavelengths and/or orthogonal polarizations. Furthermore, nonmonolithic laser arrays can assemble lasers with different wavelengths to match photoreceptor response windows in color printing systems. The individual lasing elements must be independently controlled without introducing crosstalk between adjacent lasing elements. The diode laser array, which is used with a single set of optics, should produce similarly dimensioned spots that are readily brought into registration.
A nonmonolithic laser array usually consists of a plurality of individual diode lasers mounted on a support. In applications such as laser printing, the output laser beams must be accurately spatially separated. Thus, the diode lasers of the nonmonolithic array must be supported such that accurate positioning of the lasing elements is achieved.
One approach to obtaining arrays of multiple wavelength and/or cross-polarized lasers is disclosed in U.S. Patent Application Ser. No. 08/156,227, wherein the composite array is comprised of separate single beam diode lasers attached to a common support. The support may be formed in the shape of a cross which protrudes from a base. Separate diode lasers are mounted adjacent to each inner corner of the cross. The thickness of the spacer controls the separation of lasing elements on opposite sides of the spacer. The bar of the cross controls the separation of lasing elements on the same side of the spacer. Due to practical limitations on the minimum thickness of the spacer and the bar, lasing elements mounted in this way are typically separated by 150 .mu.m, and therefore, the lasing elements are not optimally close for printing purposes. In addition, the light emitting regions of each laser chip are beneficially mounted against the spacer, thereby precluding assembly of an array of monolithic multiple beam diode lasers for which each lasing element is separately addressable.
An approach which enables assembly of an array of monolithic multiple beam diode lasers with individually addressable lasing elements is disclosed in U.S. Patent Application Ser. No. 08/096,312, now U.S. Pat. No. 5,311,536. This invention provides for nonmonolithic arrays comprised of separate multiple beam diode lasers mounted on protrusions of protruding members of a stacked support. Each diode laser can comprise multiple lasing elements that are monolithically formed in a single chip. The stacked support is implemented such that a separate conductive path is provided on each protruding member for each lasing element, thereby enabling each lasing element to be separately addressed. The protruding members are separated by spacers. However due to practical limitations on the minimum thickness of the spacer that can be used, lasing elements mounted in this way are typically separated by about 150 .mu.m, and therefore, the lasing elements are not optimally close for printing purposes.
An approach which enables assembly of separate but closely spaced lasing elements is described in U.S. Pat. No. 4,901,325 to Kato et al., which is incorporated herein by reference. FIG. 5 shows a semiconductor laser device 400 with two stacked diode lasers 402 and 404. The electrode surfaces of the diode lasers are joined together and connected to wire 412. A bottom electrode surface of diode laser 404 is soldered with a metallized surface 406 on the upper surface of mount 408. Wires 410, 412 and 414 are used to operate the semiconductor laser device 400. Although the lasing elements can be placed within 10 .mu.m of each other in this assembly, joining the electrode surfaces of each diode laser prevents use of this approach for assembling two separate monolithic arrays of multiple beam diode lasers for which each lasing element is separately addressable.
Another approach, which is disclosed in U.S. Pat. No. 4,901,325 by Kato et al., is shown in FIG. 6. Semiconductor diode lasers 426 and 428 are mounted on mounts 422 and 424, respectively. The semiconductor diode lasers are joined through an insulating layer 430. The insulating layer 430 is made of an electrically insulating material having a low thermal conductivity. Further, the active regions of the diode lasers are displaced from each other. In this way, wires 434 and 432 for electrical wiring can be bonded on one side of the chip. Although the lasing emitters can be vertically placed within 10 .mu.m of each other in this assembly, they are displaced horizontally by about 50 .mu.m. This horizontal displacement is undesirable in many applications and precludes use of this approach for assembling monolithic arrays of multiple laser emitters for which each lasing emitter is separately addressable.
In yet another approach disclosed by Kato, et al. (U.S. Pat. No. 4,901,325), an assembly of multiple separate diode lasers, each of which contains a single lasing element, is disclosed. In this approach, the diode lasers 426 and 428 are mounted side by side along either of two planar supports 422 and 424. The two supports are then brought close together as shown in FIG. 7 and attached to the base 440 without touching. Electrical connections are made separately to each diode laser. Although the inventors claim that the distance between two lasing elements on opposite supports can be as small as 10 .mu.m, lasing elements on the same support are separated by at least the width of an individual chip, e.g. 100 .mu.m or more. This separation is undesirable in many applications and precludes use of this approach for assembling arrays of lasing elements closely spaced in two dimensions.
Accordingly, all of the nonmonolithic diode laser arrays disclosed in the prior art suffer from at least one of the following problems. First, alignment of the lasing elements may involve external manipulations and accurate placement of the lasing elements on a support. Second, two supports may have to be placed in close proximity and stably held in position. Third, it may be difficult to achieve closely spaced lasing elements because edge effects or spacers limit the minimum spacing between lasing elements, especially in arrays containing more than two elements.
Thus, a need exists for methods and devices that enables close, accurate spacing of lasing elements in a nonmonolithic laser array without excessive thermal, optical, and/or electrical crosstalk. Such methods and devices are even more desirable if they permit the accurate and stable orientation of the lasing elements.